Chapter 13
Meiosis and Sexual Life Cycles
• A. The Basis of Heredity
– 1. Offspring acquire genes from parents by inheriting chromosomes.

Genes are segments of DNA.

Most genes program cells to synthesize specific enzymes and other proteins whose
cumulative action produces an organism’s inherited traits.

In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one
generation to the next.

After fertilization (fusion of a sperm cell and an ovum), genes from both parents are
present in the nucleus of the fertilized egg, or zygote.

Every living species has a characteristic number of chromosomes.
 Humans have 46 chromosomes in almost all of their cells.

Each chromosome has hundreds or thousands of genes, each at a specific location, its
locus.
– 2. Like begets like, more or less: a comparison of asexual and sexual
reproduction.

Only organisms that reproduce asexually can produce offspring that are exact copies of
themselves.

In asexual reproduction, a single individual is the sole parent to donate genes to its
offspring.
 Single-celled eukaryotes can reproduce asexually by mitotic cell division to
produce two genetically identical daughter cells.
 Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a
mass of cells by mitosis.

An individual that reproduces asexually gives rise to a clone, a group of genetically
identical individuals.
 Members of a clone may be genetically different as a result of mutation.
In sexual reproduction, two parents produce offspring that have unique combinations
of genes inherited from the two parents
•
• B. The Role of Meiosis in Sexual Life Cycles
– 1. Human cells contain sets of chromosomes.

In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.

Images of the 46 human chromosomes can be arranged in pairs in order of size to produce a
karyotype display.
 The two chromosomes comprising a pair have the same length, centromere position, and
staining pattern.
 These homologous chromosome pairs carry genes that control the same inherited characters.

Two distinct sex chromosomes, the X and the Y, are an exception to the general pattern of
homologous chromosomes in human somatic cells.

The other 22 pairs are called autosomes.

The pattern of inheritance of the sex chromosomes determines an individual’s sex.
 Human females have a homologous pair of X chromosomes (XX).
 Human males have an X and a Y chromosome (XY).

Only small parts of the X and Y are homologous.
 Most of the genes carried on the X chromosome do not have counterparts on the tiny Y.
 The Y chromosome also has genes not present on the X.

We inherit one chromosome of each homologous pair from each parent.
 The 46 chromosomes in each somatic cell are two sets of 23, a maternal set (from your mother)
and a paternal set (from your father).

The number of chromosomes in a single set is represented by n.

Any cell with two sets of chromosomes is called a diploid cell and has a diploid number of
chromosomes, abbreviated as 2n.

Sperm cells or ova (gametes) have only one set of chromosomes—22 autosomes and an X (in an
ovum) and 22 autosomes and an X or a Y (in a sperm cell).

A gamete with a single chromosome set is haploid, abbreviated as n.

Any sexually reproducing species has a characteristic haploid and diploid number of chromosomes.
 For humans, the haploid number of chromosomes is 23 (n = 23), and the diploid number is 46
(2n = 46).
– 2. Let’s discuss the role of meiosis in the human life cycle.

The human life cycle begins when a haploid sperm cell fuses with a haploid ovum.

These cells fuse (syngamy), resulting in fertilization.

The fertilized egg (zygote) is diploid because it contains two haploid sets of
chromosomes bearing genes from the maternal and paternal family lines.

As an organism develops from a zygote to a sexually mature adult, mitosis generates all
the somatic cells of the body.
•
Gametes undergo the process of meiosis in which the chromosome
number is halved.
– 3. Organisms display a variety of sexual life cycles.

Fertilization and meiosis alternate in all sexual life cycles.

In most animals, including humans, gametes are the only haploid cells.

Plants and some algae have a second type of life cycle called alternation of generations.
 This life cycle includes two multicellular stages, one haploid and one diploid.
 The multicellular diploid stage is called the sporophyte.
 Meiosis in the sporophyte produces haploid spores that develop by mitosis into the haploid
gametophyte stage.
 Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into
the sporophyte by mitosis.

Most fungi and some protists have a third type of life cycle.
 Gametes fuse to form a zygote, which is the only diploid phase.
 The zygote undergoes meiosis to produce haploid cells.
 These haploid cells grow by mitosis to form the haploid multicellular adult organism.
 The haploid adult produces gametes by mitosis.
– 4. Meiosis reduces the chromosome number from diploid to haploid.

Many steps of meiosis resemble steps in mitosis.
 Both are preceded by the replication of chromosomes.

However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II,
resulting in four daughter cells.
 The first division, meiosis I, separates homologous chromosomes.
 The second, meiosis II, separates sister chromatids.

The four daughter cells have only half as many chromosomes as the parent cell.

Meiosis I is preceded by interphase, in which the chromosomes are replicated to form
sister chromatids.

Division in meiosis I occurs in four phases: prophase I, metaphase I, anaphase I, and
telophase I.
» Prophase I

Prophase I typically occupies more than 90% of the time required for meiosis.

During prophase I, the chromosomes begin to condense.
 In crossing over, DNA molecules in nonsister chromatids break at corresponding
places and then rejoin the other chromatid.
 Each chromosome pair becomes visible as a tetrad, or group of four chromatids.
 Each tetrad has one or more chiasmata, sites where the chromatids of homologous
chromosomes have crossed and segments of the chromatids have been traded.
 The breakdown of the nuclear envelope and nucleoli take place.
» Metaphase I

At metaphase I, the tetrads are all arranged at the metaphase plate, with one
chromosome facing each pole.
» Anaphase I

In anaphase I, the homologous chromosomes separate. One chromosome moves toward
each pole, guided by the spindle apparatus.

Sister chromatids remain attached at the centromere and move as a single unit toward
the pole.
» Telophase I and cytokinesis

In telophase I, movement of homologous chromosomes continues until there is a haploid
set at each pole.
 Each chromosome consists of two sister chromatids.

Cytokinesis usually occurs simultaneously, by the same mechanisms as mitosis.

No chromosome replication occurs between the end of meiosis I and the beginning of
meiosis II, as the chromosomes are already replicated.
» Meiosis II

Meiosis II is very similar to mitosis.
 During prophase II, a spindle apparatus forms and attaches to kinetochores of each
sister chromatid.

At metaphase II, the sister chromatids are arranged at the metaphase plate.
 Because of crossing over in meiosis I, the two sister chromatids of each
chromosome are no longer genetically identical.

At anaphase II, the centomeres of sister chromatids separate and two newly individual
chromosomes travel toward opposite poles.

In telophase II, the chromosomes arrive at opposite poles.
 Nuclei form around the chromosomes, which begin expanding, and cytokinesis separates the
cytoplasm.

At the end of meiosis, there are four haploid daughter cells.
– 5. There are key differences between mitosis and meiosis.

Mitosis and meiosis have several key differences.
 The chromosome number is reduced from diploid to haploid in meiosis but is
conserved in mitosis.
 Mitosis produces daughter cells that are genetically identical to the parent and to
each other.
Three events, unique to meiosis, occur during the first
division cycle.
1. During prophase I of meiosis, replicated homologous chromosomes line up and become
physically connected along their lengths by a zipperlike protein complex, the
synaptonemal complex, in a process called synapsis. Genetic rearrangement between
nonsister chromatids called crossing over also occurs. Once the synaptonemal complex
is disassembled, the joined homologous chromosomes are visible as a tetrad. X-shaped
regions called chiasmata are visible as the physical manifestation of crossing over.
Synapsis and crossing over do not occur in mitosis.
2.At metaphase I of meiosis, homologous
pairs of chromosomes align along the
metaphase plate. In mitosis, individual
replicated chromosomes line up along the
metaphase plate.
3.At anaphase I of meiosis, it is homologous
chromosomes, not sister chromatids, that
separate and are carried to opposite poles of
the cell. Sister chromatids of each replicated
chromosome remain attached. In mitosis,
sister chromatids separate to become
individual chromosomes.
 Meiosis I is called the reductional division
because it halves the number of
chromosome sets per cell—a reduction from
the diploid to the haploid state.
• C. Origins of Genetic Variation
 What is the origin of genetic variation?
 Mutations are the original source of genetic
diversity.
 Once different versions of genes arise through
mutation, reshuffling during meiosis and
fertilization produce offspring with their own
unique set of traits.
– 1. Sexual life cycles produce genetic variation among
offspring.

•
•
•
Three mechanisms contribute to genetic variation:
1. Independent assortment of chromosomes.
2. Crossing over.
3. Random fertilization.

Independent assortment of chromosomes contributes to genetic variability due to the random
orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I.
 There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal
chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the
paternal chromosome.

Each homologous pair of chromosomes segregates independently of the other homologous pairs
during metaphase I.

Therefore, the first meiotic division results in independent assortment of maternal and paternal
chromosomes into daughter cells.

The number of combinations possible when chromosomes assort independently into gametes is 2n,
where n is the haploid number of the organism.
 If n = 3, there are 23 = 8 possible combinations.
 For humans with n = 23, there are 223, or more than 8 million possible combinations of
chromosomes.

Crossing over produces recombinant chromosomes, which combine genes inherited
from each parent.

Crossing over begins very early in prophase I as homologous chromosomes pair up gene
by gene.
 For humans, this occurs an average of one to three times per chromosome pair.

Crossing over, by combining DNA inherited from two parents into a single
chromosome, is an important source of genetic variation.
 The random nature of fertilization adds to the
genetic variation arising from meiosis.
 Any sperm can fuse with any egg.
 The ovum is one of more than 8 million possible
chromosome combinations.
 The successful sperm is one of more than 8 million
possibilities.
• The resulting zygote could contain any one of
more than 70 trillion possible combinations of
chromosomes.
– 2. Evolutionary adaptation depends on a population’s genetic variation.

Darwin recognized the importance of genetic variation in evolution.
 A population evolves through the differential reproductive success of its variant members.
 Those individuals best suited to the local environment leave the most offspring, transmitting
their genes in the process.

This natural selection results in adaptation, the accumulation of favorable genetic variations.

If the environment changes or a population moves to a new environment, new genetic combinations
that work best in the new conditions will produce more offspring, and these genes will increase.

Sex and mutation continually generate new genetic variability.

Although Darwin realized that heritable variation makes evolution possible, he did not have a theory
of inheritance.

Gregor Mendel, a contemporary of Darwin’s, published a theory of inheritance that supported
Darwin’s theory.
However, this work was largely unknown until 1900, after Darwin and Mendel had both been dead
for more than 15 years
•